Certain spectrometers cover a wide spectral band with moderate spectral resolution. However, for gas detection and measurement, only certain narrow spectral lines need to be detected, but they must be sensed with very high spectral resolution. Spectral resolution of 20,000 VAX is not an uncommon requirement.
Prior attempts to cover three spectral lines have involved three separate spectrometers, three slits, three collimators, three gratings, three imaging systems, and three focal plane arrays (“FPA”s) behind a common foreoptic. In a prior spectrometer, common collector optics image radiation onto a first dichroic beamsplitter, which reflects a first portion of the radiation and transmits a portion of the radiation. The transmitted portion is incident on a second dichroic beamsplitter, which reflects a second portion of the radiation and transmits a third portion. The first, second, and third portions are incident on a first collimator, a second collimator, and a third collimator, respectively, which image the first, second, and third portions on a first grating, a second grating, and a third grating, respectively. A first imaging system, a second imaging system, and a third imaging system then image the first, second, and third portions reflected from the first, second, and third gratings, respectively, on a first FPA, a second FPA, and a third FPA. Here, a first FPA may be silicon, and the second and third FPAs may be mercury cadmium telluride.
In this spectrometer, three different first order gratings are required. For example, the gratings have about one micron of pitch, and are positioned at about eighteen degrees off the Littrow condition, which increases shadowing and decreases throughput. This gross triplication of hardware has size, weight, and cost impacts. For example, three million detectors may be purchased and only 0.6 million detectors may be actually used.